Fiber reinforced materials are relatively brittle compared to metallic materials. The more brittle the fibers are, the brittler will be the fiber composite material. Similarly the fiber composite material will also be rather brittle if the matrix material and the fiber is brittle and/or if the bonding surface between brittle fiber and the matrix is too well adhering.
On the other hand, only relatively brittle fibers and relatively brittle matrix materials are usable in the range of high temperatures and very high temperatures, namely, in the range from 250.degree. C. to 1400.degree. C. depending on the combination of materials. Further, it is absolutely necessary that a high strength adhesion is assured at the interface between the fiber surface and the matrix material because without such high strength bonding it would not be possible to transmit the necessary shearing loads between the load carrying fibers and the load transmitting matrix material. As an extreme illustration, without such high strength bonding the fibers could be pulled out of the matrix material. However, this desirable high adhesion has the disadvantage that a crack in the brittle matrix material extending substantially perpendicularly to the longitudinal extension of the fiber, causes a high locally concentrated notch strain loading in the fiber. As a result, the fiber breaks without being able to transfer the strain around the crack tip and by this to develop a so called crack stopper effect.